Integrated electro-optic frequency comb generator

ABSTRACT

An integrated electro-optic frequency comb generator based on ultralow loss integrated, e.g. thin-film lithium niobate, platform, which enables low power consumption comb generation spanning over a wider range of optical frequencies. The comb generator includes an intensity modulator, and at least one phase modulator, which provides a powerful technique to generate a broad high power comb, without using an optical resonator. A compact integrated electro-optic modulator based frequency comb generator, provides the benefits of integrated, e.g. lithium niobate, platform including low waveguide loss, high electro-optic modulation efficiency, small bending radius and flexible microwave design.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.62/898,051, filed Sep. 10, 2019, which is hereby incorporated byreference in its entirety.

TECHNICAL FIELD

The present invention relates to optical devices and electro-opticfrequency comb generators, and in particular to an integratedelectro-optic frequency comb generator.

BACKGROUND

An electro-optic modulator (EOM) is an optical device in which anelectro-optic element is used to modulate a beam of light using anelectrical, e.g. voltage, signal. The modulation may be imposed on thephase, frequency, amplitude, or polarization of the beam. Modulationbandwidths extending into the gigahertz range are possible with the useof high bandwidth modulators. The electro-optic effect is the change inthe refractive index of the E/O material in the electro-optic elementresulting from the application of a DC or alternating electric field.The refractive index change is caused by forces that distort theposition, orientation, or shape of the molecules constituting the E/Omaterial. The simplest kind of E/O material comprises a crystal, such aslithium niobate, whose refractive index is a function of the strength ofthe local electric field. Accordingly, if lithium niobate is exposed toan electric field, light will travel through it more slowly or quicklydepending on the direction of the field. Moreover, the phase of thelight leaving the crystal is directly proportional to the length of timethe light takes to pass therethrough. Therefore, the phase of the laserlight exiting an EOM can be controlled by changing the electric field inthe crystal.

The electric field can be created by placing a parallel plate capacitoracross the crystal. Since the field inside a parallel plate capacitordepends linearly on the potential, the index of refraction dependslinearly on the field, and the phase depends linearly on the index ofrefraction, the phase modulation must depend linearly on the potentialapplied to the E/O material. The voltage required for inducing a phasechange of π is called the half-wave voltage Vπ.

Intensity modulation (IM) modulates light using an RF modulation signalat frequency ω, with an amplitude about V_(π), of that modulator, whereV_(π) is the minimum voltage required to change the intensity from aminimum to a maximum in the IM, i.e. changing the relative phase betweenthe two arms of a Mach-Zehnder interferometer by π. The IM can receive acontinuous wave (CW) optical signal and can generate a sinusoidalvarying intensity profile at the output.

Phase modulation (PM) is a modulation pattern that encodes informationas variations in the instantaneous phase of a carrier wave. The phase ofa carrier signal is modulated to follow the changing voltage level(amplitude) of modulation signal. The peak amplitude and frequency ofthe carrier signal remain constant, but as the amplitude of theinformation signal changes, the phase of the carrier changescorrespondingly. The analysis and the final result (modulated signal)are similar to those of frequency modulation. A very common applicationof EOMs is for creating sidebands in a monochromatic laser beam. Asideband is a band of frequencies higher than or lower than the carrierfrequency, containing power as a result of the modulation process. Thesidebands carry the information (modulation) transmitted by the signal.The sidebands consist of all the Fourier components of the modulatedsignal except the carrier. All forms of modulation produce sidebands.

An optical frequency comb is a coherent light source composed ofmultiple phase-locked optical carriers with equidistant frequencyspacing. Frequency comb generators are important for a wide range ofapplications from telecommunication to sensing.

Since EOMs generate phase-locked optical side-bands with equidistantspacing defined by the modulation frequency, they can be used for thegeneration of optical frequency combs. Conventionally, generatingelectro-optic frequency combs is possible using cascaded electro-opticintensity and phase modulators, typically comprising bulk modulatorsmade of lithium niobate. By cascading an intensity and a few phasemodulators connected through optical fibers, a spectrally flat frequencycomb can be generated through purely electro-optic modulation with aconversion efficiency close to one due to the lack of resonators.However, the drawbacks of this method include narrow comb width, highmicrowave driving power requirement due to multiple phase shifters andamplifiers, complex microwave components for tuning microwave phases andhigh optical insertion loss due to interconnecting discrete intensitymodulators and phase modulators.

The frequency comb generators based on cascaded intensity andphase-modulators generate comb lines by combining intensity and phasemodulation. These generators so far are based off chip and areinefficient, expensive, power hungry and have limited comb bandwidth.

An object of the present invention is to overcome the shortcomings ofthe prior art by providing an integrated on-chip comb generatorproviding efficient, inexpensive, low-power and wide bandwidth combgeneration.

SUMMARY OF THE INVENTION

Accordingly, the present invention relates to an optical deviceincluding a substrate, a device layer on the substrate, and a pluralityof waveguide-based, electro-optic modulators connected in series by awaveguide structure in the device layer.

In some embodiments, the plurality of electro-optic modulators areconfigured to receive light from at least one continuous wave lightsource, and generate a plurality of optical frequencies, said opticaldevice comprising a frequency comb generator.

In some embodiments, each of the plurality of electro-optic modulatorsincludes an RF electrode, a first electro-optic modulator coupled inseries by said waveguide structure to a second electro-optic modulator,the RF electrode of the first electro-optic modulator and the RFelectrode of the second electro-optic modulator coupled electrically toeach other or shared between the first electro-optic modulator and thesecond electro-optic modulator.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail with reference to theaccompanying drawings which represent preferred embodiments thereof,wherein:

FIG. 1A is a schematic of a multiple RF signal driven cascaded amplitudeand phase modulator series that is used to generate a frequency comb inthe optical domain;

FIG. 1B is a schematic of a single RF signal driven cascaded amplitudeand phase modulator series that is used to generate a frequency comb inthe optical domain;

FIG. 2A is an example of a comb generator with cascaded amplitude andphase modulators on chip;

FIG. 2B is a side view of the comb generator of FIG. 2A;

FIG. 3 is a multi-element electrode drive variation of an integratedcomb generator with a single PM cascaded with an IM;

FIG. 4 is a comb generator in which the waveguides are wound backthrough the second electrode gap of the PM to effectively double thephase modulation strength;

FIG. 5A is a single electrode driven by a single RF source with singlepass PMs;

FIG. 5B is a single electrode driven by a single RF source with two passPMs;

FIG. 5C is a cross-sectional view of the comb generator of FIG. 5A or5B;

FIG. 6A illustrates a multi-pass differential drive comb generator;

FIG. 6B is a multi-pass single electrode comb generator variation drivenby a single RF source;

FIG. 6C illustrates a further variation of a multi-pass differentialdrive comb generator;

FIG. 7 is an example of comb source and modulator co-integration conceptwhere comb source and modulators are integrated on the same platform;and

FIG. 8 illustrates a possible implementation of an integrated combsource and modulators integrated on the same platform.

DETAILED DESCRIPTION

While the present teachings are described in conjunction with variousembodiments and examples, it is not intended that the present teachingsbe limited to such embodiments. On the contrary, the present teachingsencompass various alternatives and equivalents, as will be appreciatedby those of skill in the art.

An integrated electro-optic frequency comb generator may include anultralow loss integrated thin-film lithium niobate or lithium tantalateplatform, which may be fabricated in accordance with the methodsdisclosed in WO 2018/031916 filed Aug. 11, 2017 by Wang et al., which isincorporated herein by reference. The platform enables low powerconsumption comb generation spanning over a wider range of opticalfrequencies. The compact integrated electro-optic modulator basedfrequency comb generator, provides the benefits of an integrated lithiumniobate or lithium tantalate platform including low waveguide loss, highelectro-optic modulation efficiency, small bending radius, and flexiblemicrowave design.

With reference to FIGS. 1A and 1B, the integrated frequency combgenerator 1, comprises one or more integrated electro-optic intensity oramplitude modulators (IM) 2, and one or more phase modulators (PM) 3 ₁to 3 _(n) configured to provide maximum modulation efficiency, ideallyprovided on a single chip 4 as a continuous waveguide structure. The IM2 may be first modulated by an RF modulation signal at frequency ω, withan amplitude about V_(π) of that modulator, where V_(π) is the minimumvoltage required to change the intensity from a minimum to a maximum inthe IM 2, i.e. changing the relative phase between the two arms of aMach-Zehnder interferometer by π. The IM 2 receives a continuous wave(CW) optical signal, e.g. a center wavelength at 1550 nm, from a CWlight source 6, and optionally passed through a polarizing opticalelement (polarizer) 7 to ensure all of the light is at a samepolarization. The modulation generates a sinusoidal varying intensityprofile, e.g. a pure tone, in the output from the IM 2. The intensitymodulated light is then sent through the at least one PM 3, preferably aplurality of PM's 3 ₁ to 3 _(n), each of which operates at frequency ω,that defines the spectral separation of the comb lines or the wavelengthchannel spacing, e.g. 10-100 GHz, to generate multiple symmetricalsidebands in the frequency spectrum on each side of the centerfrequency, space apart by frequency ω. Each PM 3 ₁ to 3 _(n) may bedriven by a common or phase locked RF source (RF1, RF2, RF3) that isphase matched to the incoming intensity modulated light so that thephase modulation spectrally broadens the pulses generated in the IM 2,as illustrated in FIG. 1A. In some variations, such as that depicted inFIG. 1A, the lengths of the optical path between devices allow RF1, RF2,and RF3 to be substantially identical, in other variations the phasesare separately tuned to match the delays caused by those lengths of theoptical paths between the devices. In some variations, such as thatdepicted in FIG. 1B, a single RF source is used, and the RF signalpasses through the IM 2 and each of the at least one PM 3 in a cascadedmanner in a similar fashion that the optical signal passes through thecascaded devices, in the same direction (as indicated) and having thesame effective path lengths. This may be achieved by using commonelectrodes spanning multiple devices, or via signal lines coupling theelectrodes of each cascaded device to the next. This results in a broadand flat comb spectrum that has near unity optical conversionefficiency. The comb may then be coupled off the chip 4 or may befurther processed on the same chip 4. The order of IM 2 and PM's 3 ₁ to3 _(n) may be reversed, e.g. the CW laser 6 may initially pass light tothe PM's 3 ₁ to 3 _(n) and then to the IM 2. By integrating the IM 2 andthe PM's 3 ₁ to 3 _(n), respectively, e.g. on a thin-film lithiumniobate (LN) chip 4, optical insertion loss is greatly reduced betweenthem and many more geometries for efficient modulation become possible.The CW light source 6 and the polarizing optical element 7 may also beon the same LN chip 4 or one or more separate chips optically coupled tothe LN chip 4. It should be noted that in designs using multiple RFsources, such as that depicted in FIG. 1A, the phases between these RFsources should be at the right values, which implies tuning of theoptical waveguide paths and/or expensive RF phase shifters need to beused for each driver port, increasing complexity, whereas for the singleRF source design, such as that depicted in FIG. 1B, at most the lengthsof the electrodes and/or RF signal lines should be tuned appropriately.

The chip-scale design allows several innovative features. First is thetight bending radii, e.g. a radius of 3 μm to 2 mm, preferably 10 μm to500 μm, and more preferably 20 μm to 200 μm, enables the waveguides tobe folded back around, cross and point in different directions reducingthe size to only a small area of the chip. Second is that each activeelement can be dramatically reduced in size due to the increasedefficiency and high-level of integration, enabling a plurality of PM'sto be located in close proximity, thereby generating additionalsidebands, i.e. a wide spectrum bandwidth. Third, the ultralow losswaveguides enable devices with very long effective lengths to beconstructed without introducing excessive optical waveguide insertionloss. Fourth, the co-integration of microwave transmission line andmultiple waveguide elements enables new design geometries thatdramatically increases comb generation efficiencies and reducesmicrowave driver complexities.

FIG. 2A illustrates an example of an integrated frequency comb generator21 including connecting waveguides with tight bending radii, ashereinbefore defined. The comb generator 21 includes at least one IM 25,providing the function of IM 2, comprising an input waveguide or port 22optically coupled to a first coupler 23, e.g. a Y-splitter or a 2×2coupler with one arm terminated, for splitting an input optical signalinto first and second sub-beams, which propagate along first and secondarms 26 and 27, respectively, to a second coupler 28, e.g. a Y-splitteror 2×2 coupler, for recombining, e.g. interfering, the first and secondsub-beams for output an IM output waveguide or port 29. Each of thefirst and second arms 26 and 27 may comprise single mode waveguidesection or a combination of single mode narrow waveguide sections, e.g.400 nm to 1000 nm wide, with a cross sectional area <3 μm², preferablyless than 1 μm², and multimode wide waveguide sections, 1000 nm to 4000nm wide and a cross sectional area of >0.5 μm² and <10 μm². The singleand multimode sections may include non-trivial guiding structures, suchas splitters, bends, and multimode interferometers (MMI). Ideally, thenarrow waveguide sections may only support one TE mode and one TM modewith optical propagation loss <0.6 dB/cm, and optical propagationloss >1 dB/cm for higher modes. The wide waveguide sections support morethan one TE mode and more than one TM mode with optical propagation loss<0.6 dB/cm for all modes.

The multimode sections may be significantly longer than the single modesections, e.g. commonly by a factor of 10 to 100; figures are not toscale. The multimode sections may include simple structures, e.g. astraight line and potentially shallow bends. The multimode sections andthe single mode sections are connected with tapers, which may bedesigned such that only the fundamental mode of the multimode waveguideis excited. Ideally, multimode sections are provided adjacent to, besideor below, a signal electrode 35 and ground electrodes 36 and 37 foremploying the electro-optic nonlinearity of the waveguide material,thereby defining electro-optic sections of the waveguide structure, andthe single mode sections comprise curved sections defining connectingsections of the waveguide structure connecting the multimode, i.e.electro-optic, sections without any electrodes adjacent thereto.

The IM output waveguide 29 may be optically coupled to a first tightcurved, i.e. tight bending radii as above, waveguide 40 ₁, whichoptically couples the IM 21 to a first PM section 41 ₁ of a plurality ofPM sections 41 ₁ to 41 _(n), providing the functions of one or more PMs3 ₁ to 3 _(n). The first PM section 41 ₁ includes a first PM waveguide42 ₁, which may comprise a single mode waveguide section or acombination of single mode narrow waveguide sections, and multimode widewaveguide sections, as hereinbefore defined. The multimode sections andthe single mode sections are connected with tapers, which may bedesigned such that only the fundamental mode of the multimode waveguideis excited. Ideally, multimode sections are provided adjacent to, besideor below, signal electrode 45 ₁ and first and second ground electrodes46 ₁ and 47 ₁.

Particular examples of such tapers would include linear tapering of thewaveguide width, cubic tapering of the waveguide width or exponentialtapering, as well as other nonlinear tapering methods. The taperingshould be gradual enough to allow modes to be adiabatically convertedfrom the single mode to the fundamental TE or TM mode of the multimodesection without excessive tapering loss or excitation of optical modesother than the fundamental TE and TM modes.

The output of the first PM waveguide 42 ₁ may be optically coupled to asecond tight curved, i.e. tight bending radii, connecting waveguide 40₂, which optically couples the first PM section 41 ₁ to a second PMsection 41 ₂ of the plurality of PM sections, providing the functions ofone or more of PMs 3 _(i) to 3 _(n). Each PM section 41 ₁ to 41 _(n) maybe a separate PM or a portion of a combined PM with adjacent PMsections. The second PM section 41 ₂ includes a second PM waveguide 42₂, which may comprise a single mode waveguide section or a combinationof single mode narrow waveguide sections, and multimode wide waveguidesections, as hereinbefore defined. The multimode sections and the singlemode sections are connected with tapers, which may be designed such thatonly the fundamental mode of the multimode waveguide is excited.Ideally, multimode sections are provided adjacent to, beside or below,signal electrode 45 ₂ and first and second ground electrodes 46 ₂ and 47₂. The second ground electrode 47 ₁ from the first PM 41 ₁ may be thesame or shared with the first ground electrode 46 ₂ from the second PM41 ₂.

The output of the second section PM 41 ₂ may be optically coupled toanother PM or PM section or to a comb output waveguide or port 49. Thefirst and second tight curved waveguides 40 ₁ and 40 ₂, i.e. tight bendradii as above, enable the first PM section 41 ₁ to be folded backbeside the IM 25, and the second PM section 41 ₂ to be folded backbeside the first PM section 41 ₁, whereby the first and second arms 26and 27 and the PM waveguides 42 ₁ and 42 ₂ may be parallel to eachother, and all of the electrodes 35, 36, 37, 45 _(1-n), 46 _(1-n), and47 _(1-n) may extend parallel to each other separated by a sufficientdistance so as to eliminate or at least limit interference therebetween.The first and second tight curved connecting waveguides 40 ₁ and 40 ₂are curved around so that the aspect ratio of the comb generator 21 maybe more square. Parallel electrodes are ideal for PM sections 41 ₁ to 41_(n) in which the crystal axis extends in the plane of the device layer11, but for PM sections 41 ₁ to 41 _(n) in which the crystal axisextends out of the plane of the device layer 11, e.g. Y-cut, theelectrodes 35, 36, 37, 45 _(1-n), 46 _(1-n), and 47 _(1-n) may run inany direction.

The difference in black level for the shaded regions in the signalelectrodes 35 and 45 _(1-n), and the ground electrodes 36, 37, 46_(1-n), and 47 _(1-n), indicates different electrode polarity. Theelectrodes are formed either in a capacitive fashion or configured in atransmission line design. In the transmission line design the microwavetravels together with the light where their group velocity is roughlymatched. The illustrated IM 25, first PM 41 ₁ and second PM 41 ₂ maycomprise X or Y-cut (or any angle therebetween) Lithium Niobate (LiNbO₃or LN), Lithium Tantalate (LT) or other electro-optic material with anelectro-optic constant >10 pm/V, such as ferroelectric materials, designincluding the central signal electrodes 35 and 45 _(n), respectively,adjacent to the outer edges of the first and second arms 26 and 27, andthe PM waveguides 42 ₁ and 42 ₂, respectively, whereby the electricfield is oriented along the X or Y axis, i.e. parallel to the devicelayer 11; however, a Z-cut LN or LT design with the signal electrode 35,and 45 _(1-n) and one of the ground electrodes over top of the first andsecond arms 26 and 27, or the PM waveguides 42 ₁ and 42 ₂, respectively,extending in any direction, e.g. parallel, perpendicular or any angletherebetween, whereby the electric field is oriented along the z axis,i.e. perpendicular to the device layer 11, is also within the scope ofthe invention. Ideally, the electro-optic material in the waveguidestructure is oriented such that the crystalline axis with the largestelectro-optic coefficient (Z-axis) is parallel, e.g. X or Y cut, to theplane of the device layer 11 with the electrodes 35, 45 _(1-n), 36, 37,46 _(1-n), and 47 _(1-n) disposed adjacent to the waveguide structure oneither side thereof configured to generate an electric field along theZ-axis or oriented such that the crystalline axis with the largestelectro-optic coefficient (Z-axis) is perpendicular, e.g. Z cut, to theplane of the device layer 11 with the electrodes 35, 45 _(1-n), 36, 37,46 _(1-n), and 47 _(1-n) disposed over top of or under the waveguidestructure configured to similarly generate an electric field along theZ-axis.

Any other waveguide material, e.g. silicon, and electrode control fortransmitting an electronic modulation signal to the optical signal iswithin the scope of the invention. Preferably, the waveguides comprisingthe input waveguide 22, the first coupler 23, the first and second arms26 and 27, the second coupler 28, the IM output waveguide 29, the firstand second tight curved waveguides 40 ₁ and 40 ₂, and the first andsecond PM waveguides 42 ₁ and 42 ₂ are comprised of thin film lithiumniobate or lithium tantalate, which may be fabricated in accordance withthe methods disclosed in WO 2018/031916 filed Aug. 11, 2017 by Wang etal.

With reference to FIG. 2B, ideally, the comb generator 21 is formed in adevice layer 11 on a substrate 20, including a lower cladding layer 12and a handle layer 13. In a preferred embodiment, the waveguides 22, 23,26, 27, 28, 29, 40 _(1-n), and 42 _(1-n) may be comprised ofelectro-optics materials (Pockels materials) with an electro-opticconstant >10 pm/V, such as single crystal Lithium Niobate (LiNbO₃ or LN)or Lithium Tantalate (LiTaO₃ or LT), and the device layer 11 with thesubstrate 20 is comprised of a thin-film electro-optic material oninsulator structure, such as Lithium Niobate on insulator (LNOI)structure (or Lithium Tantalate on insulator structure (LTOI)),including a silicon dioxide (SiO₂) lower cladding layer on a silicon(Si) handle layer. Note that the substrate 20 can be other materialssuch as quartz, sapphire, fused silica. The lower cladding layer 12 canbe any planarized material that has a lower refractive index than thewaveguide material, including, LN or LT and air (suspended structures).An upper cladding layer 14 with lower refractive index than thewaveguide, e.g. LN or LT, material, e.g. an upper SiO₂, may also beprovided covering the modulator structure, i.e. first and second arms 26and 27, and first and second couplers 23 and 28 in the device layer 11.However, other suitable waveguide materials exhibiting electro-opticeffect with an electro-optic coefficient >10 pm/V, such as galliumarsenide (GaAs), indium phosphide (InP) and barium titanate (BTO,BaTiO₃), are also within the scope of the invention.

FIG. 3 illustrates a generic design for a multi-element IM/PMconfiguration for the electro-optic frequency comb generator 21′.Utilizing the tight bending radius of a plurality of curved connectingwaveguides 40 ₁ to 40 _(n) of integrated thin film waveguide material,e.g. lithium niobate, extending between PM waveguides 42 ₁ to 42 _(n) ofa plurality of PM sections 41 ₁ to 41 _(n) may be constructed on a smallchip 20 and packed in a small area, with a plurality or all of theelectrodes 45 _(1-n), 46 _(1-n), and 47 _(1-n) extending parallel toeach other, rather than extending over excessive length, spaced apart bya sufficient distance so as to eliminate or at least limit interferencetherebetween. Although various electrode configurations are possible forY and Z cut PM waveguides 42 ₁ to 42 _(n). The PM sections 41 ₁ to 41_(n) may be treated as a single PM 3 or a plurality of PM's 3 ₁ to 3_(n) and may be driven by a common RF source 50 transmitting an RFsignal, e.g. at frequency ω, with an amplitude about V_(π), of that PM 3₁ to 3 _(n), which is split N ways, where N is the number of PM's or PMsections 41 ₁ to 41 _(n) employed. In some implementations, the RFsignals delivered to each signal electrode are phase shifted by anappropriate amount to compensate for the path length the optical signaltravels including along the curved connecting waveguides 40 ₁ to 40_(n), i.e. in such a manner that the modulation of the cascaded set ofPMs has a cumulative effect. The arrow in the picture indicates thedirection of the microwave driving field, which is tuned to be the samedirection of light propagation, also shown. Note that this directionwill be reversed if the input and output port of the light is swapped tokeep optical and microwave directions the same. Similar to the previousexample (FIG. 2A), this design needs the optical signal and the RFsignal for each PM section 41 ₁ to 41 _(n) to be phase matched, i.e. byadjusting carefully the delay between each set of microwave electrodes35, 36 and 37 caused by the length of the transmission line from thecommon RF source 50 to each PM section 41 ₁ to 41 _(n) through adjustingthe length of the optical waveguide sections 40 ₁ to 40 _(n) and 42 ₁ to42 _(n) or the phase between each RF driving signal. In some variations,where the number N of cascaded PM stages permits, rather than a set of NRF signals being provided to the N PMs, a single RF source is used inconjunction with RF signal lines (or bent electrodes such as thosedepicted in FIG. 6B below) between each PM for conveying the RF signalfrom device to device, similar to that depicted in FIG. 1B.

FIG. 4 shows a generic design for an multi-element IM/PM configurationfor an electro-optic frequency comb generator 21″ with the opticalwaveguides passing through each PM section 41 ₁ to 41 ₅ twice instead ofone time as the FIG. 3, thereby doubling the accumulated phase-shift.Utilizing the small bending radius of a plurality of curved waveguides40 ₁ to 40 _(n) of integrated thin film waveguide material, e.g. lithiumniobate, extending between PM waveguides 42 ₁ to 42 _(n) a plurality ofPM sections 41 ₁ to 41 _(n) may be constructed on a small chip 20 andpacked in a small area, with a plurality or all of the electrodes 45_(1-n), 46 _(1-n), and 47 _(1-n) extending parallel to each other,rather than extending over excessive length. Although various electrodeconfigurations are possible for Y and Z cut PM waveguides 42 ₁ to 42_(n). The PM sections 41 ₁ to 41 _(n) are driven by a common RF source50 transmitting an RF signal, e.g. at frequency ω, which is split Nways, where N is the number of PM sections 41 ₁ to 41 _(n) employed. Insome implementations, the RF signals delivered to each signal electrodeare phase shifted by an appropriate amount to compensate for the pathlength the optical signal travels including along the curved connectingwaveguides 40 ₁ to 40 _(n), i.e. in such a manner that the modulation ofthe cascaded set of PMs has a cumulative effect. The arrow in thepicture indicates the direction of the microwave driving field, which istuned to be the same direction of light propagation (also shown). Notethat this direction will be reversed if the input and output port of thelight is swapped to keep optical and microwave directions the same.Similar to the previous example, this design needs the optical and RFsignals to be phase matched, i.e. by adjusting carefully the delaybetween each microwave electrode through adjusting the length of theoptical waveguide or the phase between each RF driving signal. Inaddition the RF elements, such as splitters and delays, maybeco-integrated on the chip 20 or operating as standalone elementsoff-chip. That said, in some variations, where the number N of cascadedPM stages permits, rather than a set of N RF signals being provided tothe N PMs, a single RF source is used in conjunction with RF signallines (or bent common electrodes such as those depicted in FIG. 6Bbelow) between each PM for conveying the RF signal from device todevice, similar to that depicted in FIG. 1B.

The end of the final PM waveguide, e.g. 42 ₆, is connected to a returnor feedback waveguide 51, which extends back to the first PM section 41₁, to pass along side of the first signal electrode 45 ₁ as a firstdouble pass PM waveguide 52 ₁, e.g. on the opposite side of and parallelto the first PM waveguide 42 ₁. A first double pass curved waveguide 53₁ is connected at the end of the first double pass PM waveguide 52 ₁ forfolding the waveguide direction back through the second PM section 41 ₂to pass alongside the second signal electrode 45 ₂ as a double pass PMwaveguide 52 ₂, e.g. on the opposite side of and parallel to the secondPM waveguide 42 ₂. The first double pass curved waveguide 53 ₁ isconcentric and parallel with the second curved waveguide 40 ₂ with adifferent radius of curvature.

The length of the feedback waveguide 51 has to be chosen very carefullyand may include an active or passive delay element, such that the lightin the first double pass PM waveguide 52 ₁ is in phase with the light inthe first PM waveguide 42 ₁ but with a π phase shift for devices withcrystal axis that extend in-plane of the device layer, e.g. device layer11, and a 0 phase shift for z-cut devices, i.e. with crystal axes thatextend out of the plane of the device layer, e.g. device layer 11. Thisis important because the PM waveguides 42 _(1-n) and the double pass PMwaveguides 52 _(1-n) pass the same signal electrodes 45 _(1-n) but thefields in the two gaps are opposite. If the length of the feedbackwaveguide 51 is not chosen correctly, then the second pass could, in theworst case, remove all the acquired phase and destroy all modulation.

Subsequent double pass PM waveguides, e.g. 52 ₂ to 52 _(n), pass alongside of subsequent signal electrodes 45 ₂ to 45 _(n) e.g. on theopposite side of and parallel to the corresponding PM waveguide 42 ₂ to42 _(n). Subsequent double pass curved waveguide 53 ₂ to 53 _(n) areconnected at the end of the corresponding double pass PM waveguide 52 ₂to 52 _(n) for folding the waveguide direction back through the next PMsection 41 ₂ to 41 _(n). The double pass curved waveguides 53 ₂ to 53_(n) are concentric and parallel with the curved waveguides 40 ₂ to 40_(n) with different radiuses of curvature, i.e. alternating betweenlarger on the outside and smaller on the inside of the curved waveguides40 ₂ to 40 _(n). The end of the last double pass PM waveguide, e.g. 52₆, is connected to the comb output port or waveguide 49.

The double pass PM waveguides 52 ₁ to 52 _(n) may be comprised of singlemode waveguides or a combination of single mode and multimodewaveguides, as hereinbefore defined with reference to the PM waveguides42 ₁ to 42 _(n). The advantage with the double pass PM waveguides 52 ₁to 52 _(n) is that the modulation strength for the same number of PMsections 41 ₁ to 41 _(n) will be doubled. The drawback is that thewaveguide would need to cross each other once, e.g. the feedbackwaveguide 51 and the comb output waveguide 49, which usually is not aserious problem and can be mitigated by low loss waveguide crossdesigns.

FIGS. 5A to 5C illustrate two design variations of an electro-optic combgenerator 61, which includes only one common RF electrode 85 for both anIM 62 and a PM 63, e.g. at frequency ω, that defines the spectralseparation of the comb lines or the wavelength channel spacing, e.g.10-100 GHz, with an amplitude about V_(π) of the IM 62. FIG. 5Aillustrates a single IM 62, providing the impulse modulationfunctionality as in the IM 2 cascaded with the single PM 63, providingphase modulation functionality as in a single PM 3, although additionalPM's and IM's are possible. As illustrated in FIG. 1B, this variationutilizes a single RF electrode 85 common to both the IM 62 and the PM 63to pass the RF signal from the IM 62 to the PM 63. FIG. 5B illustrates adesign in which the waveguides are folded back through the second halfof the PM 63 to effectively double the phase modulation strength, i.e.increase the number of sidebands, at the expense of limited possible RFdriving frequencies. These designs dramatically simplifies the requiredRF signal input, i.e. reduced to only one RF signal input port, at theexpense of a preferred fixed value of RF input power to both the IM'sand PM's, instead of independently providing different amounts of RFinput power to the IM's and the PM's, as in the comb generators 21, 21′and 21″.

The comb generator 61 includes an input waveguide or port 72 opticallycoupled to a first coupler 73, e.g. a Y-splitter or a 2×2 coupler withone arm terminated, for splitting an input optical signal into first andsecond sub-beams, which propagate along first and second arms 76 and 77,to a second coupler 78, e.g. a Y-splitter or 2×2 coupler, forrecombining, e.g. interfering, the first and second sub-beams for outputan IM output waveguide or port 79. Each of the first and second arms 76and 77 may comprise single mode waveguide section or a combination ofsingle mode narrow waveguide sections, as hereinbefore described, andmultimode wide waveguide sections, as herein before described. Theconnecting, i.e. single mode, sections may include non-trivial guidingstructures, such as the first and second couplers 73 and 78. Themultimode sections may be significantly longer than the single modesections, e.g. commonly by a factor of 10 to 100, Figure not to scale.The multimode sections may include simple structures, e.g. a straightline and potentially shallow bends. The multimode sections and thesingle mode sections are connected with tapers, which may be designedsuch that only the fundamental mode of the multimode waveguide isexcited. Ideally, multimode sections are provided adjacent to, beside orbelow, signal electrode 85 and ground electrodes 86 and 87.

The IM output waveguide 79 may be optically coupled to the PM 63. The PM63 includes a first PM waveguide 82, which may comprise a single modewaveguide section or a combination of single mode narrow waveguidesections, and multimode wide waveguide sections, as hereinbeforedefined. The multimode sections and the single mode sections areconnected with tapers, which may be designed such that only thefundamental mode of the multimode waveguide is excited. Ideally,multimode sections are provided adjacent to, beside or below, the signalelectrode 85 and first and second ground electrodes 86 and 87,respectively.

The PM waveguide 82 may include one or more tight curved waveguides,e.g. first and second tight curved waveguides 88 and 89, enabling the PMwaveguide 82 to be folded back to extend along the other side of thesignal electrode 85, whereby the sections of the PM waveguide 82,ideally multimode sections, extend parallel to each other on oppositesides of the electrode 85. The difference in black level for the shadedregions in the electrodes 85, 86 and 87 indicates different electrodepolarity. The electrodes 85, 86 and 87 are formed either in a capacitivefashion or configured in a transmission line design. In the transmissionline design the microwave travels together with the light where theirgroup velocity is roughly matched. In the illustrated embodiments ofFIGS. 5A and 5B, the IM 62 and the PM 63 may be driven with onemicrowave source 60 configured to transmit one RF signal at frequency ω,that defines the spectral separation of the comb lines or the wavelengthchannel spacing, e.g. 10-100 GHz, with an amplitude about V_(π) of theIM 62. The separated optional electrode 85 a, 86 a and 87 a are DCelectrodes for DC biasing in the IM 62 via a separate DC power source.The DC biasing can also be achieved by either electro-optic or othermeans such as thermo-optic or piezoelectric tuning. The microwave powermay be configured so that it induces about a π phase shift in the IM 62while inducing other values of phase shifts in the PM 63. In both FIGS.5A and 5B, the devices are broadband for input optical frequencies, butFIG. 5B is generally narrow band for microwave driving frequencies. Thisis because the optical waveguide sections of PM waveguide 82 that windsback need to be phase matched to the microwave electrodes 85. This isdone by carefully choosing the length of the PM waveguide 82 so that theoptical delay is an integer multiple of the microwave half-wavelength inthe electrodes 85, 86 and 87. An output waveguide or port 90 is providedat the end of the PM waveguide 82.

The illustrated IM 62 and PM 63 may comprise X-cut Lithium Niobate(LiNbO₃ or LN), Lithium Tantalate (LT) or other electro-optic materialwith an electro-optic constant >10 pm/V, design including the centralsignal electrodes 85 adjacent to the outer edges of the first and secondarms 76 and 77, and PM waveguide 82, respectively; however, a Z-cut LNor LT design with the signal electrode 85 and one of the groundelectrodes over top of the first and second arms 76 and 77, or the PMwaveguide 82, respectively, is also within the scope of the invention.Any other waveguide material, e.g. silicon, and electrode control fortransmitting an electronic modulation signal to the optical signal iswithin the scope of the invention. Preferably, the waveguides comprisingthe input waveguide 72, the first coupler 73, the first and second arms76 and 77, the second coupler 78, the IM output waveguide 79, the first,and the PM waveguide 82 are comprised of thin film lithium niobate orlithium tantalate, which may be fabricated in accordance with themethods disclosed in WO 2018/031916 filed Aug. 11, 2017 by Wang et al,which is incorporated herein by reference.

Ideally, the comb generator 61 is formed in a device layer 111 on asubstrate 120, including a lower cladding layer 112 and a handle layer113, as in FIGS. 2B and 5C. In a preferred embodiment, the waveguides72, 73, 76, 77, 78, 79, and 82 may be comprised of single crystalLithium Niobate (LiNbO₃ or LN) or Lithium Tantalate (LiTaO₃ or LT), andthe substrate 120 is comprised of a Lithium Niobate on insulator (LNOI)structure (or Lithium Tantalate on insulator structure (LTOI)),including a silicon dioxide (SiO₂) lower cladding layer 112 on a silicon(Si) handle layer 113. Note that the substrate 120 can be othermaterials such as quartz, sapphire, fused silica. The lower claddinglayer 112 can be any planarized material that has a lower refractiveindex than the waveguide, e.g. LN or LT, material, including air(suspended structures). An upper cladding layer 114 with lowerrefractive index than the waveguide, e.g. LN or LT, material, e.g. anupper SiO₂, may also be provided covering the modulator structure, i.e.first and second arms 76 and 77, and first and second couplers 73 and 78in the device layer 111. However, other suitable waveguide materialsexhibiting electro-optic effect with an electro-optic coefficient >10pm/V, such as gallium arsenide (GaAs), indium phosphide (InP) and bariumtitanate (BTO, BaTiO₃), are also within the scope of the invention. Itshould be noted that variations depicted in FIGS. 5A and 5B, in which asingle RF source is shared between the IM and the PM, multiple RFsources or expensive RF phase shifters are avoided, decreasingcomplexity.

FIG. 6A illustrates a design for a differential drives comb generator61′ in which the IM 62 is similar to the IM 62 in FIG. 5A, but two RFsignals from two parallel RF electrodes 95 a and 95 b with the samefrequency, but opposite phase, are used on the PM waveguide 82 from theleft and right side. The second RF electrode 95 b may be disposedbetween the second ground electrode 87 and a third ground electrode 97all extending parallel to each other. The first and second tight curvedwaveguides 88 and 89 may be included to wrap the PM waveguide 82 back toextend between the second RF electrode 95 b and the third groundelectrode 97, as in FIG. 5B, or the third tight curved waveguide 89 maybe excluded, as in FIG. 5A, whereby the PM waveguide 82 is passeddirectly by the second RF electrode 95 b in sequence with the first RFelectrode 95 a. The RF signals from the RF electrodes 95 a and 95 b mayinclude differential voltages +V and −V about 180° out of phase which ispreferable to some applications that has a differential RF driver. Asdescribed above, in some variations the lengths of curved waveguides 88and 85 and 89 are tuned to increase the cumulative modulation effect. Itshould be noted that in the variation depicted in FIG. 6A, in which asingle RF source is shared between the IM 62 and the top portion of thePM 63, multiple RF sources or expensive RF phase shifters (for drivingthese devices) are avoided, decreasing complexity.

FIG. 6B is a multi-pass single electrode comb generator 61 b variationin which the IM 62 is similar to the IM 62 in FIGS. 5A and 6A, but asingle RF signal drives a bent RF electrode 95 a, 95 b, 95 c in a PM 63b which also includes a bent first ground electrode 86 a, 86 b, 86 c. Awaveguide 82 in the PM 63 b emerges from the IM 62 between a firstportion of the RF electrode 95 a disposed between a first portion of thefirst ground electrode 86 a and a second ground electrode 87. A thirdportion of the RF electrode 95 c is disposed between a third portion ofthe first ground electrode 86 c and the second ground electrode 87 allextending parallel to each other and the first portion of the RFelectrode 95 a and the first portion of the first ground electrode 86 a.The first and second tight curved waveguides 88 and 89 may be includedto wrap the PM waveguide 82 back to extend between the third portion ofthe RF electrode 95 c and the second ground electrode 87, and to extendbetween the first portion of the RF electrode 95 a and the second groundelectrode 87, as in FIG. 5B, and a third tight curved waveguide 85 maybe included to wrap the PM waveguide 82 back again this time to extendbetween the third portion of the RF electrode 95 c and the third portionof the first ground electrode 86 c. The first ground electrode 86 a, 86b, 86 c and the RF electrode 95 a, 95 b, 95 c, are each bent such thatthere is a connection portion or second portion of each 86 b, 95 bconnecting the first portions 86 a, 95 a to the third portions 86 c, 95c thereof. In the variation depicted in FIG. 6B, these second portions86 b, 95 b are spaced apart from the first and third curved waveguides88, 85 and cross the PM waveguide 82 at right angles. As describedabove, in some variations the lengths of curved waveguides 88, 89, and85, and the lengths of the second portions of the RF electrode 95 b andthe first ground electrode 86 b are tuned to optimize the cumulativemodulation effect. It should be noted that in the variation depicted inFIG. 6B, in which a single RF source is shared between the IM 62 and thePM 63 b, multiple RF sources or expensive RF phase shifters (for drivingthese devices) are avoided, decreasing complexity.

FIG. 6C illustrates a further variation of a multi-pass differentialdrive comb generator 61 c in which, similarly to FIG. 6A, two RF signalsfrom two parallel RF electrodes 95 a and 95 b with the same frequency,but opposite phase, are used on the PM waveguide 82, but from thedirection, left to right, to match the direction of the optical signals.The first RF electrode 95 a is disposed parallel to a first groundelectrode 86, and the second RF electrode 95 b is disposed between asecond ground electrode 87 and a third ground electrode 97 all extendingparallel to each other. A first curved waveguide 88 a is included towrap the PM waveguide 82 back around the first ground electrode 86 to asecond curved waveguide 88 b which wraps the PM waveguide 82 to extendbetween the first RF electrode 95 a and the second ground electrode 87.A third curved waveguide 88 c wraps the PM waveguide 82 back along themiddle of the second ground electrode 87, where there is substantiallyno electric field to affect a modulation of the optical signals passingthrough the PM waveguide 82. Next, a fourth curved waveguide 88 d wrapsthe PM waveguide 82 to extend between the second RF electrode 95 b andthe third ground electrode 97, while a fifth curved waveguide 88 e wrapsthe PM waveguide 82 back around the third ground electrode 97 to a sixthcurved waveguide 88 f which wraps the PM waveguide 82 to extend betweenthe second RF electrode 95 b and the second ground electrode 87. The RFsignals from the RF electrodes 95 a and 95 b may include differentialvoltages +V and −V about 180° out of phase which is preferable to someapplications that has a differential RF driver. As described above, insome variations the lengths of curved waveguides 88 a-88 f are tuned tooptimize the cumulative modulation effect. It should be noted that inthe variation depicted in FIG. 6C, in which a single RF source is sharedbetween the IM 62 and the top portion of the PM 63 c, multiple RFsources or expensive RF phase shifters (for driving these devices) areavoided, decreasing complexity.

FIG. 7 illustrates an example of a multi-wavelength component 100 on asingle chip 20 or 80 including a frequency comb generator, e.g. 21, 21′,21″, 61, 61′, 61 b, or 61 c to generate a frequency comb, a wavelengthdivision multiplexing (WDM) filter 101 to filter particular lines of thecomb into spatially-separated, individual constituent wavelength opticalsignals, and a plurality of modulators 102 ₁ to 102 _(n) configured tomodulate each individual wavelength optical signal on the same materialplatform. A lithium niobate (or other aforementioned material) thin-filmplatform is ideal for this because it possess all the necessary opticalelement and electro-optic properties to complete such functionalities.

FIG. 8 illustrates an example of such a layout for the applicationdescribed in FIG. 7. The comb generator, e.g. illustrated comb generator61 although any of the aforementioned comb generators 21, 21′, 21″, 61,61′, 61 b, or 61 c is possible, generates an optical frequency comb froma CW input laser 6. The generated comb is then transmitted to the WDMfilter 101, e.g. a plurality of ring resonators 111 ₁ to 111 _(n); whichmay also be replaced with any suitable WDM filter, such as an arrayedwaveguide grating, to filter out and spatially separate individualoptical wavelength signals. The individual optical wavelength signalsare then filtered and modulated through the plurality of individualmodulators 102 ₁ to 102 _(n), which may be simple IMs or in-phase andquadrature (IQ) modulators.

The foregoing description of one or more embodiments of the inventionhas been presented for the purposes of illustration and description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed. Many modifications and variations are possiblein light of the above teaching. It is intended that the scope of theinvention be limited not by this detailed description, but rather by theclaims appended hereto.

We claim:
 1. An optical device comprising: a substrate; a device layeron the substrate; and a plurality of waveguide-based, electro-opticmodulators connected in series by a waveguide structure in the devicelayer, each electro-optic modulator including an RF electrode, theplurality of electro-optic modulators including: a first electro-opticmodulator and a second electro-optic modulator coupled in series by saidwaveguide structure, and a continuous integrated RF electrode integratedwithin the device layer and extending through the first electro-opticmodulator and the second electro-optic modulator, a first length of thecontinuous integrated RF electrode being the RF electrode of the firstelectro-optic modulator, extending through the first electro-opticmodulator, but not extending through the second electro-optic modulatorand a second length of the continuous integrated RF electrode being theRF electrode of the second electro-optic modulator, extending throughthe second electro-optic modulator, but not extending through the firstelectro-optic modulator, wherein the plurality of electro-opticmodulators and the waveguide structure are configured to receive lightfrom at least one continuous wave light source, and generate a pluralityof optical frequencies, said optical device comprising a frequency combgenerator.
 2. The optical device according to claim 1, wherein the firstelectro-optic modulator comprises an intensity modulator (IM) andwherein the second electro-optic modulator comprises a phase modulator(PM).
 3. The optical device according to claim 2, wherein the RFelectrodes of the plurality of electro-optic modulators are configuredto receive at least one continuous wave RF input signal.
 4. The opticaldevice of according to claim 2, wherein each of at least oneelectro-optic modulator of the plurality of electro-optic modulatorsincludes two RF electrodes coupled to a pair of RF signal lines tosupport differential drive signals.